.body {
    height: 100%;
    display: grid;
    grid-template-rows: auto 1fr;
    grid-template-areas: 
        "h"
        "m";
}

.header {
    grid-area: h;
    height: 70px;
    box-sizing: border-box;
    padding: 0 20px;
    display: flex;
    align-items: center;
}

.main {
    grid-area: m;
    box-sizing: border-box;
}